1. Field of the Invention
The present invention relates to a method of manufacturing a nitride semiconductor device.
2. Description of the Related Art
In recent years, nitride semiconductor devices containing GaN, InN, AlN or the like, are heavily used as electronic device materials for blue and green light emitting diodes, light emitting devices such as blue-violet semiconductor lasers and high speed transistors operable at high temperatures.
A manufacturing method is suggested, where a growth substrate is replaced with a metal substrate having high heat dissipation property after forming a semiconductor device layer on the growth substrate.
For example, as one method, the method includes the steps of forming a nitride semiconductor layer on a sapphire substrate, decomposing neighbor of a boundary surface between the sapphire substrate and the nitride semiconductor layer, by irradiating laser light of ultraviolet region from reverse side of the sapphire substrate, and separating the nitride semiconductor layer from the sapphire substrate. (for example, Unexamined Japanese Application No. 2000-101139)
Moreover, as another method, the method includes the steps of forming a stripping layer having lower band-gap energy than a growth substrate on the growth substrate composed of the GaN, irradiating laser light having a energy higher than the band-gap energy of the stripping layer and lower than the band-gap energy of the growth substrate, after forming a nitride semiconductor layer on the stripping layer, and separating the nitride semiconductor layer from the growth substrate. (for example, Unexamined Japanese Application No. 2005-93988)
Here, in the method where the stripping layer is formed on the growth substrate, since the nitride semiconductor layer is formed on the stripping layer, a material of the stripping layer is necessarily same material of the nitride semiconductor layer. In this case, a composition of the stripping layer is necessarily to be formed of an alloy composition close to the nitride semiconductor layer, and the stripping layer is necessarily to be formed into thin film, for decreasing crack by strain for nitride semiconductor layer and defects such as threading dislocations. That is, since the composition of the stripping layer is close to the nitride semiconductor layer, the band-gap energy of the stripping layer approximates the band-gap energy of the nitride semiconductor layer and the growth substrate. Since, the stripping layer formed into thin film, the laser light easily penetrates through the stripping layer.
As described above, as the photon energy of the laser light is to be about the same as the band-gap energy of the nitride semiconductor layer, and amount of the laser light penetrating the stripping layer increases, the laser light could be absorbed in the nitride semiconductor layer (for example, an active layer formed in an light emitting device), corresponding to the composition of the nitride semiconductor layer and amount of the defects in grain. That is, characteristics of the nitride semiconductor device (optical characteristics and electrical characteristics) may be degrade by the absorption of the laser light causing damages to the nitride semiconductor layer.
Generally, metals (such as palladium, nickel and platinum), used for a material of a p-side ohmic electrode provided in the nitride semiconductor device, tend to easily absorb short wavelength light included in a visible light region. Therefore, when the photon energy of the laser light is to be about the same as the band-gap energy of the nitride semiconductor layer and a wavelength of the laser light irradiated to the stripping layer is relatively short, the laser light is easily absorbed in the electrode such as the p-side ohmic electrode. That is, the electrode easily generates heat and the electrical characteristics of the nitride semiconductor device may be degraded.